1. Field of the Invention
The present invention relates to a grain oriented electrical steel sheet not having a glass film (a forsterite film) and particularly to a grain oriented electrical steel sheet having a high magnetic flux density and an ultra low iron loss and remarkably excellent workability, such as slittability, cuttability and punchability, and a process for producing the same.
2. Description of the Prior Art
Grain oriented electrical steel sheets are used mainly as an iron core material for transformers and other electrical equipment and should be excellent in magnetic properties, such as inductions and an iron loss property.
In order to obtain good magnetic properties, it is necessary to highly arrange the &lt;001&gt; axis which is an easily magnetizable axis in the direction of rolling. Further, sheet thickness, grain size, specific resistance, film, etc. are also important because they have a great influence on the magnetic properties.
The orientation of grains has been remarkably improved by a method characterized by a high reduction ratio in final cold rolling wherein AlN and MnS are used as an inhibitor, so that, at the present time, it has become possible to provide steel sheets having a magnetic flux density close to the theoretical value. On the other hand, film properties and workability in addition to magnetic properties are important to the use of grain oriented electrical steel sheets by customers. In general, grain oriented electrical steel sheets are treated with a film having a double layer structure comprising a glass film formed in the final box annealing and an insulating film. The glass film is composed mainly of forsterite (Mg.sub.2 SiO.sub.4) that is a product of a reaction of MgO as an annealing separator with SiO.sub.2 formed during decarburization annealing. This ceramic film is hard and highly resistant to abrasion and has a significant adverse effect on durability of tools used in working of electrical steel sheets, such as slitting, cutting and punching. For example, when grain oriented electrical steel sheets having a glass film are subjected to punching, there occurs abrasion of molds and the occurrence of burr in the sheet at the time of punching becomes significant after effecting the punching about several thousand times, which leads to problems of use. For this reason, it becomes necessary to effect regrinding of molds or replacement of the molds with new molds. This lowers the working efficiency in the working of iron cores by customers and incurs an increase in the cost. With respect to the magnetic properties of the electrical steel sheets, although an improvement in the iron loss can be certainly attained by virtue of the tension of the film, in some forming conditions an increase in the thickness of the film or other unfavorable phenomenon unfavorably gives rise to a lowering in the magnetic flux density due to the presence of non-magnetic substances. For this reason, in the case of thick materials wherein improvement of the iron loss by the tension of the film is expected, or in the case where the iron loss can be improved by the division of the magnetic domain using other means, grain oriented electrical steel sheets not having a glass film are desired rather than grain oriented electrical steel sheets having a glass film because of the above-described problem.
Especially, in recent years, techniques using optical, mechanical and chemical means have been developed for refinning the magnetic domain, which enables the iron loss to be improved without the tension of the glass film, and it has become apparent that the grain oriented electrical steels sheet not having a glass film are advantageous over those having a glass film by virtue of the absence of an adverse effect of an internal oxide layer of the glass film which causes a pinning phenomenon with respect to the movement of the domain wall in the magnetization. For this reason, there is an ever-increasing demand for the development of a grain oriented electrical steel sheet having a high magnetic flux density and not having a glass film which is important when various working conditions used by customers are taken into consideration.
A process for producing a grain oriented electrical steel sheet not having a glass film is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 53-22113. In this process, the thickness of an oxide film is brought to 3 .mu.m or less in the decarburization annealing, particular alumina containing 5 to 40% of a hydrous silicate mineral powder is used as an annealing separator, and final annealing is effected with this annealing separator coated on the steel sheet. According to the description of the specification, this method reduces the thickness of the oxide film, enables an easily removable glass film to be formed by virtue of the incorporation of the hydrous silicate mineral and provides a steel sheet having a metallic gloss. As a method for inhibiting the formation of a glass film by using an annealing separator, Japanese Unexamined Patent Publication (Kokai) No. 56-65983 discloses a technique wherein an annealing separator comprising aluminum hydroxide and, incorporated therein, 20 parts by weight of an additive for removing impurities and 10 parts by weight of an inhibitor is coated on a steel sheet to form a thin glass film having a thickness of 0.5 .mu.m or less. Further, Japanese Unexamined Patent Publication (Kokai) 59-96278 proposes an annealing separator comprising Al.sub.2 O.sub.3, which is less reactive with SiO.sub.2, as an oxide layer formed in the decarburization annealing and MgO which has an activity lowered by sintering at a high temperature of 1,300.degree. C. or above. According to the description of the specification, the proposed annealing separator can inhibit the formation of forsterite.
All the above-described prior art techniques are based on a low-quality grain oriented electrical steel sheet having a magnetic flux density as low as 1.88 Tesla or less usually called "orient core", and no technique for stably providing a grain oriented electrical steel sheet having a high magnetic flux density contemplated in the present invention has hitherto been developed in the art.